<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>垂直对齐</title>
  <style type="text/css">
      .container {
          border: 1px solid #ccc;
          margin: 10px;
          padding: 10px;
          height: 200px;
      }
      .item {
          background-color: #ccc;
      }

      .container-1{
          text-align: center;
      }

      .container-2 {
          position: relative;
      }
      .container-2 .item {
          width: 300px;
          height: 100px;
          position: absolute;
      }

      .container-3 {
          position: relative;
      }
      .container-3 .item {
          width: 200px;
          height: 80px;
          position: absolute;
      }

      .container-4 {
          position: relative;
      }
      .container-4 .item {
          width: 100px;
          height: 50px;
          position: absolute;
      }
  </style>
</head>
<body>
<div class="container container-1">
  <span>一段文字</span>
</div>

<div class="container container-2">
  <div class="item">
    this is item
  </div>
</div>

<div class="container container-3">
  <div class="item">
    this is item
  </div>
</div>

<div class="container container-4">
  <div class="item">
    this is item
  </div>
</div>
<h3>垂直居中</h3>
<li>inline 元素: line-height 的值等于 height 值</li>
<li>absolute 元素: top: 50% + margin-top 负值</li>
<li>absolute 元素: transform(-50%, -50%)</li>
<li>absolute 元素: top, left, bottom, right = 0 + margin: auto</li>
<ul></ul>   </div>
</body>
</html>
